The mouth (oral cavity) contains resident and non-resident microflora. The resident microflora includes microorganisms that are able to establish a more or less permanent residence on the oral surfaces. These bacteria are mainly localized on the tongue, the buccal mucosa, and the teeth while the gingiva, lips, checks, palate, and floor of the mouth only support a very sparse microflora.
On the tongue and the buccal mucosa, the natural resident microflora includes microorganisms selected from Streptococcus, Veillonella, Bacteroides, and Haemophilus. On the teeth, Streptococcus, Lactobacillus and Actinomyces predominate but a variety of Gram positive and negative cocci and rods can be also found.
For example, Frandsen et al. showed that S. sanguis predominates on the buccal mucosa but its primary habitat is the surface of teeth, that S. gordonji grows in the mature supragingival plaque, and that S. oralis and S. mitis grow in the initial dental plaque (Oral Microbiol. Immunol., 6, 129-133, 1991). Strains belonging to the mutans group are localized on teeth (S. criscetus, S. downei, S. ferus, S. macacae, S. mutans, S. rattus, S. sobrinus). Strains belonging to the S. milleri group predominate in dental abscesses (S. anginosus, S. constellatus, S. intermedius) (Bentley et al., Int. J. System. Bacter. 1991, 41, 487-494; Wood et al., The Genera of Lactic Acid Bacteria, Blackie Academic and Professional, Chapman & Hall, W. H. eds., 1995).
Many of these microorganisms are innocuous commensal microorganisms, but a lot of them have been recognized as being the etiologic agent responsible for several diseases (Hill, M. J. and Marsh, P. D. eds. Human Microbial Ecology, 1990, CRC Press. Boca Raton Fla., USA)
Dental plaque is a film that forms on the surface of teeth consisting of bacterial cells in a matrix of extracellular polysaccharide and salivary products. Immediately after eruption, the teeth are covered with an amorphous layer of saliva, the acquired enamel pellicle (AEP), that is about 1.3 μm thick and cannot be removed by normal tooth brushing. The deposition of bacteria on teeth immediately follows the formation of the AEP and plaque becomes evident in 8-12 hours as a multi-layered structure. The first layer consists of bacteria (earliest colonizers) that attach to teeth, mainly via specific adhesion-receptor recognition, and forms a substratum for the second colonizers that adhere one to the other by analogous specific binding or by simple juxtaposition. Plaque cohesion is essentially guaranteed by three mechanisms: the presence of a salivary pellicle on the outer bacteria layer, the specific coaggregation among the different bacterial species, and the glucans synthesized by the bacteria that remain entrapped in the plaque matrix (Skopek et al., Oral Microbiol. Immunol., 2, 19-24, 1994; Kolenbrander et al., Meth. Enzymol., 253, 385-397, 1995; Hiroi et al., FEMS Microbiol Lett., 96, 193-198. 1992; Gibbons et al., Infect. Immun., 52, 555-561, 1986).
The organic acids produced by oral bacteria during the fermentation process directly cause dental caries. These acids attack the hard tissue of teeth with the consequent release of ions such as calcium, phosphate, carbonate, magnesium, fluoride, and sodium. When the pH in the oral cavity again increases to around neutrality, the saliva becomes saturated with calcium so that calcium liberation from the tooth is prevented. Among all the food residues found in the mouth, carbohydrates show the highest caries promoting effect since they are directly available for fermentation by oral bacteria.
Potentially all microorganisms that ferment sugars are cariogenic, but the primary etiological agents of coronal and root caries are the mutans streptococci because they are strong acid producers; Lactobacilli, that are highly aciduric, however, can also be implicated. In humans, S. mutans and S. sobrinus are the more cariogenic strains, and live on teeth while not colonizing the entire dentition. Their number is also less on anterior teeth than on molar teeth (Lindquist et al., Dent. Res., 69, 1160-1166, 1990). Moreover in human approximal plaque, S. mutans and S. sobrinus preferentially colonize the most caries-prone site apical to the contact area (Ahmady et al., Caries Res., 27, 135-139, 1993). A higher prevalence of S. sobrinus was also found in the molar regions compared with that of S. mutans (Lindquist et al., Caries Res., 25, 146-152, 1991).
S. mutans and S. sobrinus have been shown to attach to the pellicle of teeth mainly via specific adhesion-receptor interaction. Gibbons et a). showed that S. mutans carries an adhesion which binds to salivary components in the pellicle, while S. sobrinus cells appear to possess an adhesion which binds to glucan in the pellicle (Infect. Immun., 52, 555-561, 1986).
The transient microflora comprise exogenous bacteria that are occasionally present in the mouth, but that do not establish a permanent residence therein (even if repeated oral administrations of these bacteria are carried out). All the food bacteria, and in particular lactic acid bacteria, can be part of this transient microflora. These exogenous lactic bacteria have never been shown to be capable of directly adhering to the pellicle of teeth. Repeated administration of exogenous lactic bacteria may, however, lead to colonization of the mouth on all the oral surfaces, such as the tongue, the buccal mucosa, the gingiva, lips, cheeks, palate, floor, and the teeth. This colonization may result from attachments via specific bindings to bacteria of the resident micro flora (co-aggregation phenomena), via entrapment in the matrix of polysaccharide produced by the resident bacteria, or via adhesion to saliva proteins (especially glycoproteins).
Lactobacillus casei rhamnosus GG (ATCC53103) has been reported to colonize the mouth, most probably on the epithelium of the buccal mucosa. This strain also adheres to the epithelium of the intestinal tract (U.S. Pat. No. 5,032,399, Gorbach et al.; Micr. Ecol. In Health and Dis., 2, 295-298, 1994). By contrast L. rhamnosus does not adhere to teeth.
Japanese patent no. 4021633 (Cyconmedix KK) also reported colonization of the mouth by Lactobocillus acidophilus, most probably on the epithelium of the buccal mucosa. Many Lactobacillus acidophilus are known to also adhere to the epithelium of the intestinal tract (EPS77904; EP199535; Perdigon et al., Medicina, 46, 751-754, 1986; Perdigon et al., Immunology, 63, 17-23, 1988).
Exogenous bacteria can also produce factors that inhibit the growth of the resident microflora in the mouth. For example, EP759469 (Société des Produits Nestlé) described the use of a bacteriocin produced by Micrococcus varians for inhibiting the development of the oral pathogens S. sobrinus , S. sanguis, S. mutans, and A. viscous. 
There are several strategies to minimize the development of resident microflora of the mouth. For example, by administering commensal bacteria of the resident microflora that are not cariogenic, such as Streptococcus salivarius and/or Stomatococcus mucilaginosus, and/or repeated administration of exogenous lactic bacteria such as L. casei, L. fermentum, L. acidophilus, L. crispatus, L. gasseri, L. salivarius, L. bulgaricus, and S. salivarius (Tanzer et al., Infec. and Immunity, 48, 44-50, 1985; WO92/14475).
The application of bacteriocins is another investigated strategy which has been used to reduce tooth caries. These molecules have attracted interest as prospective anti-carie agents and as factors important in modulating colonization of the oral cavity. The anti-carie potential of applying bacteriocins comes from their potent and broad antibacterial activity against mutans streptococci and bacteria associated with dental plaque and their natural occurrence in bacteria regarded as being safe to humans (U.S. Pat. No. 5,368,845 to Colgate, and WO 94/12150 to Smithkline Beecham).
The application of milk derivatives is also of interest for the health of the mouth. Indeed, U.S. Pat. No. 5,427,769 (Nestec S.A.) describes another alternative wherein dental caries are prevented by contacting teeth with an edible composition containing micellar casein in amount sufficient to inhibit colonization by Streptococcus sobrinus. EP748591 (Société des Produits Nestlé S.A.) also reports the use of fluoridated micellar casein or its micellar subunits for treating dental caries or plaque. U.S. Pat. No. 4,992,420 (Nestec S.A.) describes treatment of the buccal cavity with kappa-caseino-glycomacropeptide derived from milk for eradicating plaque and caries.
Lactic bacteria that are not part of the resident microflora of the mouth have never been shown to be really capable of directly adhering to the pellicle of teeth. By colonizing the surface of teeth, however, such lactic bacteria could exert an inhibitory activity against the growth of the resident microflora, including oral pathogens.